Method for increasing the contrast of electrophotographic prints

ABSTRACT

A method for producing electrophotographic prints having increased contrast is disclosed. In the method, the dielectric charge retentive surface of an electrophotographic recording medium, such as dielectric coated conductive paper, is uniformly charged with charge of a first polarity. A photoconductive member disposed overlaying the charged surface area of the recording medium is illuminated with a photon image to be printed. An electrical potential is applied across the charged dielectric surface of the recording medium and the photoconductive member to withdraw charge from the charged surface of the recording medium in accordance with the conductive pattern in the photoconductor produced by illumination of the photoconductor with the photon image. The amplitude of and/or the time that the potential is applied across the charge retentive surface to withdraw the charge is proportioned to withdraw more charge from the light areas of the image than was originally deposited by the charging step. In this manner, the light areas of the charge image on the recording medium are thereby charged with an opposite polarity to the charge remaining on the dark portion of the photon image. The resultant composite charge image on the recording medium, which contains a charge image pattern of opposite polarities, is then exposed to charged toner particles of a given polarity to develop the composite charge image pattern. Increased contrast is obtained because the toner particles are attracted to the charge image pattern of one polarity and repelled from the charge image pattern of the other polarity.

g [22] 'Filed: July 7, 1969 United States Patent 1 1151 3,649,261

Dahlquist et a1. [45] Mar. 14, 1972 [54] METHOD FOR INCREASING THE 5 [57] ABSTRACT CONTRAST F i A method for producing electrophotographic prints having in- ELECTROPHOTOGRAPHIC PRINTS creased contrast is disclosed. In the method, the dielectric I charge retentive surface of an electrophotographic recording [72] Inventors John Damqlust Guy both of medium, such as dielectric coated conductive paper, is

Palo Alto Cahf' uniformly charged with charge of a first polarity. A photocon- [73] Assignee: Varian Asociates, Palo Alto, Calif. ductive member disposed overlaying the charged surface area of the recording medium is illuminated with a photon image to i be printed. An electrical potential is applied across the [211 App]. 839,344 charged dielectric surface of the recording medium and the photoconductive member to withdraw charge from the 1 charged surface of the recording medium in accordance with [52] US. Cl. ..96/l R, 96/ 1.3, 355/3 1 the conductive pattern in the photoconductor produced by il- [51] Int. Cl. ..G03g 13/22 lumination of the photoconductor with the photon image. The [58] Field of Search ..96/l,1.3,1.4 amplitude of and/or the time that the potential is applied 1 across the charge retentive surface to withdraw the charge is [56] References Cited proportioned to withdraw more charge from the light areas of I the image than was originally deposited by the charging step.

UNITED STATES PATENTS 1 In this manner, the light areas of the charge image on the 2,825,814 3/1958 Walkup ....250/49.5 ramming medium hereby charged with 96/1 polarity to the charge remaining on the dark portion of the "96/1 photon image. The resultant composite charge image on the j recording medium, which contains a charge image pattern of l 0 site polarities, is then exposed to charged toner particles 3,057,! 19 /1962 Byme et a1 ..96/1 gg polarity to develop the composite charge image pap 3l2l873 2/1964 McNaney "346/74 I tern. Increased contrast is obtained because the toner parti- 3l47679 9/1964 Schaffert cles are attracted to the charge image pattern of one polarity 2,833,648 5/1958 Walkup 2,937,943 5/1960 Walkup 3,015,304 1/1962 Carlson et al ..118/637 3,240,596 3/1966 Medley etal. ..96/1 I d H d f th h n f th m 3,268,331 8/1966 Harper ..96/1 22 e 6 mage Pa em e 3,464,818 9/1969 Woly ..96/l.3

2Claims,2Drawing Figures Primary Examiner-George F. Lesmes Assistant Examiner-John C. Cooper, 111 AttorneyLeon F. Herbert (RED LIGHT) FEW To (WHITE LIGHT) (b) J Egg/9 J r5 l/ /l I4/ /I l/ NEGATIVE PRINT j/ POSITIVE PRINT POSITIVE PRINTQZ/ NEGATIVE PRINT METHOD FOR INCREASING THE CONTRAST OF ELECTROPHOTOGRAPHIC PRINTS DESCRIPTION OF THE PRIOR ART Heretofore, it has been proposed to produce electrophotographic prints by uniformly charging an area to be printed of a dielectric charge retentive surface of an'electrophotographic recording medium with charges of a first polarity and then illuminating a photoconductive member to withdraw charges from the charged surface of the recording medium through the photoconductor in accordance with the illuminating pattern thereon. The resultant charge image, when subsequently developed by black toner particles, constitutes a positive print of the object being photographed. Such a method for producing positive electrophotographic prints is disclosed in U.S. Pat. No. 2,937,943 issued May 24, 1960.

The problem with the prior art electrophotographic prints, produced in accordance with the aforedescribed prior art method, is that either all of the charge originally deposited in the areas where charge is subsequently to be removed is not, in fact, removed or at best is merely neutralized. As a result, when the charged toner particles are applied for developing the charge image pattern on the recording medium, some of the toner particles are deposited in the area where charge has supposedly been removed. If these particles are not, in fact, attracted to this region they are not repelled therefrom such that a certain residual gray background is produced causing the resultant prints to have poor contrast.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved method for making electrophotographic prints.

One feature of the present invention is the provision, in a method for producing electrophotographic prints, of uniformly charging the charge retentive surface of the electrographic recording medium, illuminating a photoconductor adjacent the charged surface of the recording medium and applying a potential across the recording medium and photoconductor to remove'charge from the charge retentive surface in accordance with the illuminated areas of the photoconductor, such charge withdrawing potential being sufficiently great in amplitude or extending for a sufficient time to remove more charge than was originally deposited in the corresponding areas in order to produce a resultant composite charge image pattern on the recording medium having both positive and negatively charged patterns thereon, whereby toner particles used to develop the resultant composite image of opposite polarities results in the toner particles being attracted to the image portion of one polarity and being repelled from the image portion of the opposite polarity to increase the contrast of the resultant print.

Another feature of the present invention is the same as the preceding feature wherein the recording medium constitutes a dielectric coated conductive paper with the dielectric coating forming the charge retentive surface.

Other features and advantages of the present invention become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic perspective view, partly broken away, of an electrophotographic device employing the printing method of the present invention, and

FIG. 2 includes a pair of flow diagrams depicting the steps in alternate methods for formation of the electrophotographic prints according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I, there is shown an electrophotographic apparatus for practicing the method of the present invention. A dark box 1 is closed at one end by an optically transparent plate 2, as of glass. An optically transparent conductive electrode 3 is deposited on the glass plate 2. A photoconductive plate 4 is disposed over the transparent conductive electrode 3. The photoconductive plate 4 is preferably of the type comprising a continuous polycrystalline layer of a matrix of interlocking photoconductive crystals of a substance selected from the group consisting of sulphides, tellurides, selenides, and sulpho selenides of a member of the group consisting of zinc and cadmium and containing activator and coactivator proportions of members of the group consisting of halides, copper, and silver, and having a glass binder interstitially disposed of the polycrystalline matrix. Such a photoconductor is described and claimed in copending U.S. application Ser. No. 721 ,331 filed Apr. 15, 1968 and assigned to the same assignee as the present invention.

A projector 5 is provided at the other end of the dark box I from the platelike sandwich of glass 2, electrode 3 and photoconductor 4 for projecting a photon image to be printed onto the photoconductive plate 4 through the transparent plate 2 and electrode 3. The projector 5 includes an object 6 to be reproduced and a lamp 7 for projecting the photon image of the object 6 onto the photoconductor 4, and a shutter, not shown, which is opened to pass the photon image and closed to keep stray light out of the dark box 1.

The photoconductive plate 4 is disposed overlaying a strip of electrographic recoding paper 11 drawn from a supply roll 12. A conductive plate-shaped electrode 13 is disposed over the electrographic paper 11. The electrographic paper 11 comprises a thin film of dielectric, as of 4 to 8 microns thick, coated on a conductive paper backing. A suitable dielectric film is polyvinylbutyral and a suitable conductive paper has a resistivity of less than 10 Q-cm. The paper 11 has a thickness as of 75 microns. Electrographic recording paper of this type is available from Plastic Coating Corporation of Holyoak, Massachusetts.

The dielectric film of the electrographic paper 11 forms a charge retentive surface having a resistivity as of 10' Q-cm. The paper 11 is disposed with the dielectric film facing the photoconductive plate 4 and in virtual contact therewith. The paper will have a peak-to-peak surface roughness on the order of 10 microns such that when the paper is in nominal contact with the photoconductive plate 4 there is a minute airgap between the paper and the photoconductive plate 4 of about 6 to 8 microns. A variable source of electrical potential, as of 300 to 700 volts, is provided by a DC source of voltage 15 connected across a potentiometer 16. The voltage picked off by the potentiometer 16 is fed via a reversing switch 17 to the conductive transparent electrode 3 via a timing switch 18. The plate-shaped electrode 13 is grounded as is one pole of each pair of poles of the reversing switch 17. Thus, when the reversing switch is thrown to the left, a negative potential is applied across the photoconductive plate 4 and dielectric film to ground, whereas, when the reversing switch is thrown to the right, a positive potential is applied across the photoconductive plate 4 and film to ground. A floodlight 21 is provided inside the dark box 1 and includes a switch outside the box 1, not shown, for turning the light on or off, as desired.

In the group of preferred photoconductors, e.g., CdS, as aforecited, conduction is primarily by electrons, with hole conduction playing an insignificant role. Thus when light is absorbed by the photoconductor 4, an electron hole pair is created and the electron is transported by the action of the electric field either to and across the minute airgap separating the photoconductor 4 from the dielectric charge retentive surface, or, if the field is in the reverse direction the electron is transported back to the transparent conductive electrode 3 between the glass substrate 2 and the photoconductor 4. The hole can be considered stationary, and remains essentially where it was created until it is eliminated by recombination with another electron.

Light is absorbed by the photoconductor 4 according to its wavelength; blue light is absorbed very strongly, typically within a fraction of a micron of penetration into the photoconductor 4, while red light (or light which is near the edge of the absorption band of the photoconductor) is absorbed less strongly. Thus red light creates electron-hole pairs throughout the bulk of the photoconductor 4, while blue light creates pairs only near the photoconductor-electrode interface.

The actual condition mechanism depends upon complex physical parameters, such as the nature of the contacts at either side of the photoconductor 4, the homogeneity of the photoconductor 4, the electric field distribution throughout the entire system, the transport properties of the photoconductor 4 and airgap, and many others. However, without going into any conduction details, it can be seen that for electron conduction in the normal direction, i.e., with negative potential applied to electrode 3, the electrons should be created near the elecu'ode-photoconductor interface and transported across the photoconductor, into the airgap, and deposited on the dielectric-coated paper 11. For reverse polarity, i.e., with positive potential applied to electrode 3, the photoelectrons should be generated near the photoconductor airgap interface to be most effective in the conduction process.

Thus, when employing the aforecited photoconductors, such as CdS, with normal bias, i.e., biased for transport of electrons to the dielectric charge retentive surface of the recording paper 11 from the photoconductor 4, light or photons of any wavelength which will generate electron-hole pairs may be employed and the light or photons are preferably absorbed entirely by the photoconductor. When employing the aforecited photoconductors with reverse bias, i.e., biased for transport of electrons from the dielectric charge retentive surface of the paper 11 back to the photoconductor, light or photons of a wavelength or wavelengths that is not strongly absorbed is preferably employed so that the light can penetrate deeply into the photoconductor 4.

It is to be understood that although conduction within the photoconductor is that of electrons and not holes, conduction within the air gap may be due to electrons, ions, or both, and this discussion is not meant to limit the conduction mechanism to that of electrons within the airgap.

In operation, a length of the electrographic paper 11 is pulled from the roll 12 over the photoconductive plate 4. The reversing switch 17 is thrown to the left or right to connect a potential, as of 500 or +500 volts, respectively, to the conductive electrode 3 via timer switch 18. The floodlamp 21 is lighted and the timer switch 18 activated to apply the selected potential across the photon illuminated photoconductive plate 4 and electrographic paper 11, for example, for one second or less.

In this first step of the methods (a) and (a') of FIG. 2, the charge retentive film of the paper 11 is charged uniformly, either with positive charge or negative charge, as shown in steps (b) and (b), respectively. In step (b) where a positive charge is transferred to the dielectric charge retentive surface of the paper 11, the photoconductor 4 is preferably flooded with red light from light 21, which is not strongly absorbed in the photoconducton to transfer a uniform positive charge of a few nano Coulombs/cmF. In step (b) where a negative charge is transferred uniformly from the photoconductor 4 to the charge retentive surface of the paper 11, the photoconductor is flooded uniformly with white light while the bias potential is applied to transfer a uniform charge density of 30 to 50 nano Coulombs/cm. onto the charge retentive surface of paper 11.

After the paper 11 has been charged, the reversing switch 17 is thrown to the right or left, respectively, for applying an opposite polarity potential, as of +500 or -500 volts, respectively across the photoconductive plate 4 and paper 11 via timer switch 18. The timer switch 18, projector and shutter are then actuated for an exposure interval of, for example, 0.1 seconds or less, depending upon the light intensity of the photon image. During this exposure interval, the photoconductive plate is illuminated by the photon image of the object to be printed and a charge withdrawing potential is applied across the illuminated photoconductive plate 4 and paper 11 to ground. In this step, as shown in FIG. 2 (c and c), the amplitude of the charge withdrawing potential and/or the exposure time is selected to remove more charge from the charged surface of the charge retentive film of the paper 11, in the bright regions of the photon image, than was originally uniformly deposited thereon to produce a bright image charge pattern of opposite polarity than the dark image pattern which retains its original charge because in the dark regions the charge is not conducted away through the photoconductive plate 4. As a result, a composite charge image 22 of both positive and negative polarity is formed on the paper 11, as shown in FIG. 2 (c and c).

In step (c), of method (a) the photoconductor 4 is illuminated in the image pattern by white light and the positive charge withdrawing exposure is slightly larger than that typically employed to lay down a negative charge image, such as from 20 percent to percent more time is preferably employed. At any rate, the exposure time, light intensity and applied potential are such that the resultant negatively charged image pattern in the light areas of the optical image has a charge density of approximately 30 nano Coulombs/cmf", whereas the positively charged background, from which charge is not withdrawn, retains a charge density of approximately +5 nano Coulombs/cmF.

In step (c') of method (a'), the negative charge withdrawing potential is applied while the photoconductor is illuminated in the optical image pattern by light which is not strongly absorbed in the photoconductor, such as red light. The exposure time is preferably 1.5 to 3 times the normal exposure time using red light, i.e., the time to transfer negative charge from the photoconductor to the paper using red light. If blue or green light is used, exposure times of from 10 to 100 times the normal exposure time using blue or green light may be required. Such long exposure times are possible with a photoconductor that has no dark current, but most photoconductors have a finite dark current and therefore such long exposure times are not recommended. The light intensity, exposure time, and applied voltage are chosen to give a correct exposure which will give a positive charge density in the background areas of about +20 nano Coulombs/crn. and a negative image of charge density of about 30 nano Coulombs/cm..

After the composite charge image 22 has been formed in steps (c) and (c'), it is developed in steps (d (d (d,') or (d:) by any one of a number of conventional methods employing electrographic toners. In the apparatus of FIG. 1, the charge image 22 is developed by pulling the charge image bearing paper 1 1 over an inking screen roller 23 in the manner as disclosed and claimed in copending US. Pat. application Ser. No. 798,304 filed Feb. 1 l, 1969 and assigned to the same assignee as the present invention. Briefly, the electrographic paper 11 bearing the charge image pattern 22 to be developed is fed between a pair of input feed rollers 24 and thence between the rotatable cylindrical screen roller 23 and idler rollers 25 which press the charge image bearing surface of the paper 11 into contact with the screen 23 and thence between a pair of output feed rollers 26. The screen roller 23 rotates in a liquid toner reservoir 27 filled to a suitable level with electrographic toner 28. The toner 28 is carried in the mesh of the screen by capillary action or surface tension and is brought into close contact with the paper 11. As the screen rotates it applies a continuous supply of toner to the paper 11. Screen 23 and rollers 25 are preferably conductive and operated at ground potential.

The black or opaque toner particles, as of carbon, are in a colloidal suspension in a dielectric quick-drying liquid. The charge toner particles of a certain one polarity which may be positive or negative, depending upon the type of toner, are drawn to the opposite polarity portion of the composite charge image 22 and held thereto by electrostatic attraction, thereby developing same. The toner particles are repelled from the charge image portion having the same polarity as that of the toner particles, as shown in FIG. 2 in steps (11,), (d,,), (d.') and (d,') and at 29 in FIG. 1. The result is an electrographic print of increased contrast as compared with the prior art, such print may be either a positive or negative print of the object 6 to be printed. Negative prints are obtained by steps (d,) and (d whereas positive prints are obtained by steps 2) and r The original uniform distribution of charge, obtained by the first step of the method, may be deposited on the charge retentive surface of the paper 11 by any one of a number of methods. As an alternative to flooding the photoconductive plate 4 with photons, the dark current of the photoconductive plate 4 may be employed for transferring charge to the paper 11. In this embodiment, the method is essentially the same as that previously described with regard to FIGS. 1 and 2 except that the floodlight 21 is not actuated and the potentiometer 16 is set, for example, to about $700 volts to be applied via timer switch 18 across the photoconductive plate 4 and paper 11. The dark current flowing through the photoconductive plate under this applied voltage transfers the charge uniformly to the charge retentive surface. The amount of charge transferred is a function related to the product of voltage and time.

As an alternative, the charge may be applied by a conductive electrode such as a roller or by a conductive bar riding on or over the surface of the paper. Also, a corona discharge may be employed for charging the charge retentive surface of the paper 11. These and other conventional methods for charging charge retentive surfaces are described in a text entitled, Electrophotography, by Schaffert, published in 1965 by Focal Press Limited, New York, see pages 25 and 26.

Although, the method and apparatus of FIGS. 1 and 2 were described employing optical photon radiation, the term photon as used herein and in the claims is defined to include X-ray infrared, gamma ray and other types of highand lowenergy photon radiation as well as neutron, alpha, beta, or other particulate ionizing radiation. Thus, the method described herein is equally applicable to electroradiographic and infrared cameras. in the case of radiographic devices, the photoconductive plate 4 is made of a material, such as lead iodide, lead sulfide, and indium phosphide, which is rendered conductive by X-ray photons.

Also, the method of the present invention has been described, for the sake of ease of explanation, as though a uniform surface charge was deposited of a first polarity and then this charge, plus some excess charge, was withdrawn in an image pattern to yield a resultant composite image pattern of positive and negative charge patterns. This method can equally well be considered as the superposition of a uniform surface charge of a first polarity and a surface charge image pattern to be printed of a second polarity upon a common charge retentive surface area of a recording medium. Using this latter theory of explanation, the charge image of second polarity has an image portion of greater charge density and an image portion of lesser charge density than the uniform charge density of the first polarity such that the resultant composite charge image pattern obtained by superimposing the two results in an image pattern to be printed of one polarity and a second portion of a second polarity, whereby the charged toner particles of one polarity, employed for development of the image, are attracted to one portion of the composite charge image and repelled from the other portion of the composite charge image.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is: 1. A method of electrophotographically recording dual polarity charge image onto a dielectric coated web through a CdS photoconductive plate in which hole conduction is insignificant, comprising the steps of:

placing the dielectric coated web ad acent to one side of the CdS plate;

establishing an electric field through the CdS plate and the dielectric coating by placing a positive voltage on the plate relative to the web;

providing a light source for generating electron-hole pairs within the CdS plate, said light source emitting light primarily in the red portion of the spectrum which penetrates the CdS plate establishing pair generation proximate the web-plate interface;

illuminating the CdS plate with said red light source causing pair generation, the electrons generated being mobilized by the electric field to establish a uniform charge of a positive polarity on the dielectric coating;

reversing the direction of the electric field through the CdS plate and the dielectric coating;

exposing the CdS plate to the image to be recorded by a white light source causing pair generation, the electrons being mobilized by the reversed electric field to discharge the uniform charge of positive polarity on the dielectric coating from the exposed portions thereof and to establish on these exposed portions a charge of negative polarity resulting in a dual polarity charge on the dielectric coating; and

developing the dual polarity charge image with toner particles charged oppositely to that portion of the dual polarity charge image which is to be toned.

2. The method of claim 1 wherein white light is used for creating a uniform negative charge on the dielectric coating and red light is used for the imagewise exposure, said electric fields being reversed accordingly. 

2. The method of claim 1 wherein white light is used for creating a uniform negative charge on the dielectric coating and red light is used for the imagewise exposure, said electric fields being reversed accordingly. 